The use of signals of one or more structure-borne noise sensors for analyzing the combustion process in the combustion chambers of an internal combustion engine is described, for example, in German Patent Application No. DE 198 45 232. These structure-borne noise sensors have the advantage compared to a direct pressure measurement in the combustion chambers using pressure sensors that they are considerably less expensive and their installation is relatively simple. The pressure variation in the combustion chambers is recognizable mainly by an analysis of the low-frequency component of the structure-borne noise signal. This low-frequency component is in turn caused by the deformation of the engine block resulting from the compression in the combustion chamber. The signal of such structure-borne noise sensors is, however, often subject to considerable drift over a longer time.
An object of the present invention is to reliably recognize such a drift of the structure-borne noise sensor during operation of the engine.